Abstract Risky sexual behaviors have high personal, social, and financial costs. Family communication about sex can reduce risky sexual behaviors, but most studies focus only on the teen-parent dyad. High levels of extended- family involvement in childrearing and sexuality communication, such as with grandparents, aunts and uncles, older siblings and cousins, and ?fictive kin,? especially in Black and Latino families, suggest the importance of assessing this under-studied influence. Preliminary findings show significant associations between extended- family sexuality communication and teen sexual behavior, but the risk or protective effects of this communication remain unclear. To address NICHD's goal of identifying protective factors that prevent unintended pregnancy and sexually transmitted infections, this mixed-methods study proposes the first comprehensive assessment of teens' extended-family sexuality communication and its associations with sexual behavior, and includes extended-family perspectives in order to apply quantitative findings to prevention and intervention programs. The proposed study applies an established conceptual model of parent-teen sexuality communication to extended family, which recognizes both direct talk about sex and indirect (less straightforward) sexuality communication, which predict teens' sexual beliefs and behaviors. The study will investigate three aims: 1) Compare teens' sexuality communication with parents and with extended family, 2) Investigate associations between extended-family sexuality communication and teens' sexual beliefs and behaviors, and 3) Explore how extended-family members perceive sexuality communication with teens and what tools they need to support teens' sexual health. To address these aims, this study uses an explanatory sequential mixed-methods design, which includes teen surveys and extended-family interviews. Seventeen hundred male and female 11th and 12th graders from 4 schools will fill out an in-school online survey, and 30 extended-family members will be interviewed. Quantitative analyses will address Aims 1 & 2. It is hypothesized that parent and extended-family sexuality communication differ and that teens' perceptions of family members' sexuality communication and attitudes will be associated with teens' sexual beliefs and behaviors. Teens' closeness with the extended family member and values of familism are expected to shape the strength of these associations. The direction of associations will vary by whether teens talk with older or younger extended family. Qualitative analyses will guide implementation of findings from Aims 1 & 2 to support school and community-based prevention and intervention efforts. The significance of this work lies in its comprehensive focus on the full range of partners in family sexuality communication, assessing whether this communication is helpful or harmful to teens' sexual health. It contributes to public health by informing whether and how to include larger family systems, beyond the nuclear family, into prevention and intervention programs.